Increased concentrations of plasma cholesterol, LDL cholesterol, and LDL apo B have been implicated as risk factors for the development of atherosclerosis. The levels of LDL in plasma will be significantly regulated by LDL receptor activity, especially hepatic LDL receptor activity. However, LDL concentrations are also influenced by the rates of VLDL secretion and conversion to LDL. In the last several years it has become evident, that overproduction of apo B-containing particles, without an accompanying comparable increase in their fractional catabolic rate, is responsible for a large percentage of patients with hypercholesterolemia. Therefore, the proposed studies will investigate the regulation of apo B secretion in human hepatoma cells (Hep G2) and, later, in cultures of primary human hepatocytes. A study recently completed in my laboratory has shown that only a fraction of the apo B molecules synthesized by Hep G2 cells are secreted, in fact, a large proportion are rapidly degraded soon after synthesis. In addition, adding oleic acid to the culture medium had the effect of protecting nascent apo B from intracellular degradation. Thus, it appears that a post-translational mechanism is responsible for determining the number of apo B molecules secreted by the hepatocyte. It is the PI's overall hypothesis that the secretion of apo B-containing lipoproteins is regulated by processes which direct apo B to either the degradative or secretory pathways. It will be important in the understanding of the regulation of apo B secretion to define this pathway in more detail and study what factors affect it. Therefore, the specific aims of this proposal are: 1) Delineate the post- translational mechanism which regulates the proportion of newly synthesized apo B that is secreted by Hep G2 cells. 2) Determine how oleic acid alters the above degradative mechanism and protects nascent apo B from intracellular degradation. 3) Determine whether changes in cellular cholesterol availability affect the apo B degradative mechanism. 4) Determine which domains of the apo B molecule are involved in the regulation of intracellular degradation of apo B by transfecting Hep G2 cells with cDNAs coding for truncated apo B molecules.